1. Field of the Invention
The present invention relates to an optical pickup apparatus that uses a plurality of light sources that emit laser beams having mutually different wavelengths to optically read information on optical recording media having mutually different recording densities.
2. Description of the Related Art
Optical pickup apparatuses are already known that are capable of reading information from optical discs, namely from both CD-type optical discs including CDs, CD-ROMs and information recorded CD-Rs, and DVD-type playback-only optical discs including DVDs and DVD-ROMs. Optical pickup apparatuses of this kind comprise two light sources that emit laser beams of mutually different wavelengths on account of the fact that the recording densities of CD-type optical discs and DVD-type optical discs differ, and that the reflectance of CD-R discs and DVD discs is respectively different. One of these two light sources is selected in accordance with whether a disc to be read is either one of a CD-type disc and a DVD-type disc.
An objective lens and a photodetector are both used commonly, as a single unit, for a CD-type disc or a DVD-type disc, and, consequently, an optical path combination prism is provided for passing beams emitted by the two light sources on the same optical path such that each of the beams is on the same optical path from a midway point toward the disc.
An optical pickup apparatus of this kind typically performs a focusing servo with the astigmatism method and a tracking servo with the three beam method, which constitute cost-effective and simple methods. The astigmatism method is applied to beams to thereby detect a focusing error, a cylindrical lens being disposed downstream from the optical path combination prism (facing away from the optical path combination prism and toward the disc). Further, the three-beam method necessitates two sub-beams in addition to a main beam used for information reading, these two sub-beams (xc2x11 order diffracted lights) being generated by diffracting a laser beam emitted by a light source by means of a grating disposed downstream from the optical path combination prism.
As shown in FIG. 1, a photodetector, which is used in such an optical pickup apparatus, comprises: a four-section light-receiving portion 1, which is used to receive a main beam, and sub-beam light-receiving portions 2 and 3, which are disposed such that the four-section light-receiving portion 1 lies therebetween. The astigmatism method is performed by receiving a main beam, which has been astigmatized by means of a cylindrical lens, by means of the four-section light-receiving portion 1, performing a calculation involving the addition and multiplication of the four amounts of light received, and then obtaining a focusing error signal. Further, the three-beam method is performed by receiving two sub-beams, which have been produced by a grating, in sub-beam light-receiving portions 2 and 3, and then obtaining a tracking error signal from the difference between amounts of light by received sub-beams.
Also, in the photodetector shown in FIG. 1, since the main beam is received by the four-section light-receiving portion 1, it is possible to perform a tracking servo with the phase difference method. Further, a tracking servo can be properly performed such that the three-beam method is used for CD-type discs, and the phase difference method is used for DVD-type discs.
On the other hand, among DVD-type discs, DVD-RAMs are known that are capable of rewriting and recording information. DVD-RAM discs are land and groove type optical discs in which the pitch of the guide grooves themselves is larger than that of conventional optical discs, in a comparison of optical spots, in order that information can be recorded in the guide grooves (grooves) and also the guide groove intervals (lands), of substantially equal width. For this reason, when a focusing servo is performed with the astigmatism method, there is a problem that a noise component of a focusing error signal, which is generated when an optical spot crosses a track as a result of decentering of an optical disc, is larger than the noise component of a conventional disc. Also, due to the fact that land and groove type discs such as DVD-RAMs have land portions and groove portions of equal width with a view to equalizing the respective amount of light reflected from the land portions and groove portions, even if a tracking servo is to be performed with the three-beam method, since there is barely any change to the amount of light of the two sub-beams, even off-track, there is a problem that it is therefore difficult to pick up a tracking error signal.
Therefore, for a focusing servo of a DVD-RAM disc, a canceling system is recommended that uses a photoreception signal for the two sub-beams to cancel the noise component, resulting from track crossing, of the focusing error signal obtained by using the astigmatism method, and for a tracking servo of a DVD-RAM disc, the push-pull method or the differential push-pull method is recommended.
The above-described conventional optical pickup apparatus is capable of playback of both CD-type optical discs including CDs, CD-ROMs, and information-recorded CD-Rs, and of DVD-type playback-only optical discs including DVDs and DVD-ROMs. Further, it is required that information-recorded DVD-RAM discs can be played in the conventional optical pickup apparatus. However, as described above, since the focusing servo method and tracking servo method for DVD-RAMs are different from those of other DVDs, it is difficult to easily implement an optical pickup apparatus for playing either of CD-type optical discs and DVD-type optical discs including information-recorded DVD-RAM discs.
An object of the present invention is to provide an optical pickup apparatus that is correctly capable of playback of either of CD-type optical discs and DVD-type optical discs including DVD-RAM discs.
According to the present invention, there is provided an optical pickup apparatus comprising: a first light source for emitting a first laser beam having a first wavelength; a second light source for emitting a second laser beam having a second wavelength that is longer than the first wavelength of the first laser beam; a first grating having a diffraction grating formed with a first predetermined grating pitch, for passing through the first laser beam as a main-beam, and for outputting two sub-beams composed from diffracted lights of the first laser beam; a second grating having a diffraction grating formed with a second predetermined grating pitch different from the first predetermined grating pitch, for passing through the second laser beam as a main-beam, and for outputting two sub-beams composed from diffracted lights of the second laser beam; and an optical system for guiding the main beam and the two sub-beams to a recording surface of a recording medium, and for guiding the main beam and the sub-beams reflected at the recording surface of the recording medium to a photodetector to form respective spots corresponding to the main-beam and the two sub-beams reflected, on light-receiving surfaces of the photodetector, wherein assuming that a distance from the first light source to the first grating is given by a, a distance from the second light source to the second grating is given by b, the first predetermined grating pitch is given by I, the second predetermined grating pitch is given by J, the wavelength of the first laser beam is given by xcex1, and the wavelength of the second laser beam is given by xcex2,
xcex1/xcex2 greater than (Ixc2x7b)/(Jxc2x7a)
is satisfied.